The invention relates to an opto-electronic element on a substrate with an electric circuit comprising an electroconductive layer,
the opto-electronic element including a body for emitting light, during operation, in a predetermined wavelength range, and
being provided with current conductors which contact the electroconductive layer.
The invention also relates to a method of providing an opto-electronic element on a substrate with an electric circuit comprising an electroconductive layer,
the opto-electronic element comprising a body for emitting, in operation, light in a predetermined wavelength range, and
being provided with current conductors which, after providing the opto-electronic element on the substrate, electrically contact the electroconductive layer.
Opto-electronic elements comprise electroluminescent elements, for example light-emitting diodes (LEDs). Such opto-electronic elements are used as a source of white or colored light for general lighting applications, and as a source of colored or white light in signaling lamps, for example in traffic control systems, vehicles, aircraft or other transport means or transport systems. In recent years, apart from yellow and red light-emitting diodes on the basis of GaP, also efficient blue and green light-emitting diodes on the basis of GaN have been developed.
An opto-electronic element mentioned in the opening paragraph is known from German patent DE-A 44 20 698. In that patent, a description is given of an opto-electronic element comprising a body and current conductors, which opto-electronic element is connected, by means of an adapter with a socket provided with channels for allowing passage of current conductors of the opto-electronic element, via current-supply conductors of the adapter, to an electroconductive layer on a substrate. Such an adapter is used in surface-mounting techniques, for example in a so-called SMD technique (Surface Mounted Device). The use of an adapter with a socket provided with channels has the disadvantage that additional parts are necessary for positioning the opto-electronic element and for connecting it to the substrate. Another unfavorable aspect resides in that the known opto-electronic element requires additional connections. The reason for this being that the current conductors of the known opto-electronic element are connected to the current-supply conductors of the adapter which in turn are connected to the electroconductive layer on the substrate. In addition, the adapter takes up additional space, which is unfavorable if mounting of opto-electronic elements in small housings is desired. A further drawback of the known opto-electronic element is that it has a relatively poor thermal balance during operation. Another unfavorable aspect resides in that the known opto-electronic element requires additional connections. The reason for this being that the current conductors of the known opto-electronic element are connected to the current-supply conductors of the adapter which in turn are connected to the electroconductive layer on the substrate. In addition, the adapter takes up additional space, which is unfavorable if mounting of opto-electronic elements in small housings is desired. A further drawback of the known opto-electronic element is that it has a relatively poor thermal balance during operation.
It is an object of the invention to provide an opto-electronic element of the type described in the opening paragraph, which can be readily and accurately mounted on the substrate. A further object of the invention is to provide a method by means of which an optoelectronic element can be readily provided on a substrate.
To achieve this, an opto-electronic element of the type described in the opening paragraph is characterized, in accordance with the invention, in that the body of the opto-electronic element is bonded by means of an adhesive at a predetermined location onto the substrate with clearance with respect to said substrate, the body contacting the substrate via the adhesive.
The use of an adhesive has the advantage that the opto-electronic element is directly connected to the substrate and, unlike the known opto-electronic element, without employing an adapter with a socket. As a result, the use of an adapter with a socket has become redundant, which causes costs to be reduced and, in addition, has a space-saving effect. In addition, a connection formed by using an adhesive enables the opto-electronic element to be accurately positioned, during mounting, at a predetermined location on the substrate.
The method of providing an opto-electronic element on a substrate, as mentioned in the opening paragraph, is characterized in that the body of the opto-electronic element is provided by means of an adhesive at a predetermined location on the substrate with clearance with respect to said substrate, said body making contact with the substrate via the adhesive.
During mounting the opto-electronic elements on the substrate, in general, first the body is provided in the adhesive. This takes place, preferably, at a stage where the adhesive (still) allows the body to move with respect to the substrate, for example when the adhesive has not yet (fully) cured. During mounting, the opto-electronic element is provided such that the body of the opto-electronic element does not contact the substrate and that, preferably, the current conductors of the opto-electronic element are exposed with respect to the substrate. This enables great freedom in the positioning of the opto-electronic element to be obtained without the current conductors causing damage (for example scratches) in the electric circuit or the electroconductive circuit on the substrate during movement of the opto-electronic element. Such damage may give rise to defective (electrical) connections, which is undesirable. A further advantage of current conductors which are exposed with respect to the substrate during mounting the opto-electronic element, resides in that a relatively thin adhesive layer can be used and that, after positioning the opto-electronic element on the substrate, the (rebounding) current conductors do not lift the opto-electronic element from the adhesive.
The adhesive causes the body to be positioned with clearance with respect to the substrate. The adhesive also causes the opto-electronic element to remain positioned at the predetermined location on the substrate during connecting the current conductors to the substrate. After the opto-electronic element has been provided at the desired location and positioned there, the current conductors are connected to the electroconductive layer at predetermined locations. For this purpose, a force is exerted on the current conductor, during the connecting process, causing the part of the current conductor which is connected to the electroconductive layer to contact the relevant, corresponding part of the electroconductive layer. In an alternative embodiment, first a force is exerted on the current conductor, causing the distance between the current conductor and the corresponding part of the electroconductive layer to be set at a predetermined, relatively small value, whereafter the current conductor is connected to the electroconductive layer.
A preferred embodiment of the opto-electronic element in accordance with the invention is characterized in that the current conductors are connected to the electroconductive layer while being subjected to a tensile force.
The advantage of a tensile force resides in that it causes the body of the opto-electronic element to engage the adhesive under spring pressure. In this manner, a permanent intense contact between the body and the adhesive is guaranteed.
Preferably, the tensile force in the connection between the current conductors and the electroconductive layer lies in the range between 0.4 and 2.0 N.
In an alternative embodiment of the opto-electronic element in accordance with the invention, the current conductors are connected to the electroconductive layer so as to be substantially stress-free. The absence, at least to a substantial degree, of tensile force in the resulting connection between the current conductors and the substrate causes the reliability of the connection during the working life to be improved.
A further object of the invention is to provide an opto-electronic element having an improved thermal balance. To achieve this, a particularly preferred embodiment of the opto-electronic element in accordance with the invention is characterized in that the adhesive is heat-conductive. In this embodiment, the adhesive is not only used as a means for improving the positioning of the opto-electronic element but also as a so-called heat sink, which causes heat to be dissipated, in operation, from the opto-electronic element to the adhesive. The application of a heat-conductive adhesive is important, in particular, for opto-electronic elements wherein the light output is dependent upon the temperature. Besides, the use of a heat-conductive adhesive also improves the service life of the opto-electronic element. A particularly suitable type of adhesive comprises a mixture of a type of adhesive which is known per se and a heat-conductive filler. By using a relatively thin adhesive layer, also the heat dissipation of the opto-electronic element is improved. Preferably, the thickness of the adhesive layer is less than 50 xcexcm.
A further preferred embodiment of the opto-electronic element in accordance with the invention is characterized in that the substrate is provided with means for dissipating heat from the body via the adhesive to the substrate during operation. In this manner, heat originating from the opto-electronic element is effectively dissipated via the adhesive to the substrate.
The current conductors of the opto-electronic element are connected to the electroconductive layer of the substrate, preferably, by means of laser welding. Laser welding has the advantage that heat is very locally supplied to the current conductor during a relatively very short period of time. In this manner, heat is prevented from undesirably flowing to the opto-electronic element, which could cause damage to said opto-electronic element. In addition, laser welding has the advantage that the connection between the current conductor and the substrate can be accurately made at predetermined locations on the substrate. The appearance of the resulting welded joint is characteristic of laser welding.
For the substrate use can very suitably be made of a so-called metal-based printed circuit board (PCB) comprising an aluminium base layer, a glass-fiber reinforced insulation layer and a top layer of copper, wherein the electronic circuit is provided, for example, by, means of an etch treatment. The high coefficient of thermal conduction of copper enhances the dissipation of heat originating from the opto-electronic element, which reaches the substrate via the adhesive. In an alternative embodiment, the substrate comprises a metal-core base plate provided with a PCB.
Preferably, the opto-electronic element comprises a light-emitting diode. A further preferred embodiment of the opto-electronic element in accordance with the invention is characterized in that the wavelength range wherein the opto-electronic element emits light includes wavelengths above 550 nm. Particularly opto-electronic elements, for example light-emitting diodes, which emit light in this (yellow, red or infrared) wavelength range are temperature-sensitive, i.e. the light output of the opto-electronic element is temperature-dependent. In general, yellow and red light-emitting diodes are made on the basis of AlInGaP, and blue and green light-emitting diodes are made on the basis of InGaN.